Charged particle beam apparatuses have many functions in a plurality of industrial fields, including, but not limited to, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems. Thus, there is a high demand for structuring and inspecting specimens within the micrometer and nanometer scale.
Micrometer and nanometer scale process control, inspection or structuring, is often done with charged particle beams, e.g. electron beams, which are generated and focused in charged particle beam devices, such as electron microscopes or electron beam pattern generators. Charged particle beams offer superior spatial resolution compared to, e.g. photon beams due to their short wavelengths.
The charged particle beam devices used in the field of semiconductor comprise lithographic devices, inspection devices as well as CD (critical dimension) measurement and DR (defect review) devices. Typically, low voltage electron microscopy is used for semiconductor inspection and metrology to avoid charging of the semiconductor substrate and damage resulting therefrom.
However, in modern low voltage electron microscopes, aberrations limit the achievable resolution to approximately 3 nm for 1 keV electron energy and considerable effort has been done to optimize the lens aberrations, especially those of the objective lens. Lens optimization has been pushed to the limits in recent years so that aberration correction becomes more and more important.
For low energy applications, chromatic aberration is dominant. The diameter of the aberration disc of the chromatic aberration in the Gaussian image plane of an objective is proportional to the relative energy width ΔE/E of the charged particle beam. It is already known to utilize monochromators, in order to further increase the resolution. Thereby, the energy width ΔE of the electron beam, which is processed subsequently by the downstream electron-optical imaging system, can be reduced.
Wien filters are known as monochromators for charged particles wherein an electrostatic dipole field and a magnetic dipole field are superposed perpendicularly to each other. For example, EP 03028694.2 (Frosien et al.) describes a Wien filter monochromator with a superimposed quadrupole field that allows for improved reduction of chromatic aberration.
However, with increasing resolution requirements not only chromatic aberration but also spherical aberration has to be reduced or compensated, i.e. corrected, in order to fulfill increasing resolution requirements. The spherical aberration of the total system is dominated by the impairments of the objective lens. In “Chromatic and Spherical Aberration Correction in the LSI Inspection Scanning Electron Microscope” by K. Honda and S. Takashima, JEOL News, Vol. 38, No. 1, 2003, pages 36 to 40, a multipole corrector capable of combined chromatic and spherical aberration correction is described. The corrector consists of 4 stages of 12-pole-pin multipoles and additional coils for the generation of quadrupole fields. It is explained by the authors themselves that this correction system is so complicated that the SEM image may be completely lost due to small errors.
It is therefore an object of the present invention to overcome at least in part the disadvantages associated with the prior art. Preferably, a charged particle system capable of chromatic and spherical aberration correction should be provided. More preferably, the design of such a system is of relatively low complexity.